Human beings have been consuming yeast bread for 6000 years and fermenting yoghurt for almost the same time. Although each fermentation process is different from the others, the common point is that food undergoes a chemical change and a new product is obtained from the ingredients used. But how does fermentation that produces some life-sustaining food occur? For an answer, firstly we should get to know yeasts and bacteria acting as yeasts.
Yeast is a living organism, but it is neither an animal nor a plant. It can’t be seen with naked eyes. There are plenty of them on flowers or fruits, especially the ones containing sugar. It was Louis Pasteur who in 1857 discovered that yeast is a microorganism. Paul Nurse, another scientist, was awarded the Nobel prize for his research on cell structure and cell cycle of yeasts. Yeasts, on which science puts great emphasis, reproduces quickly in an environment with no oxygen, plenty of sugar, and a temperature between 0 and 45 degrees Celsius.
They do it by absorbing organic carbon, that is, sugar. Just like humans and some mammals, their DNA is within a nucleus. When the cell has grown enough, it splits into two. Another way of reproduction is what we call ‘budding’. In this method, a small bud is formed on the parent cell, forming a new cell. Now let’s take a look at fermentation. Fermentation is, in fact the process of producing energy. Let’s think about bread making. Water is mixed with flour. Then, some yeast is added into the mixture and stirred well.
From then on, yeasts begin to absorb the sugar, or glucose, in flour by using their membrane. Inside sugar is carbon. By using sugar, yeast produces energy called ATP. During the process, carbon dioxide, ethyl alcohol, and water are released. When bread is placed in oven, alcohol is driven off by the heat of the baking whereas carbon dioxide causes the dough to rise. It is the carbon dioxide gas that gives the bread its spongy texture.
The bacteria used to ferment yoghurt are known as yoghurt cultures. They are also unicellular. Yoghurt cultures differ from yeasts in that they are more primitive and can reproduce quite faster. They have no membrane around their DNA and their nucleus is in cytoplasm. Let’s take a closer look at how these bacteria turn milk into yoghurt. One spoonful of yoghurt is enough to turn milk into yoghurt since it contains millions of yoghurt cultures in it.
These cultures reproduce quickly, spreading through milk. And by using sugar, they produce lactic acid. Lactic acid causes milk proteins to sink to the bottom. Milk becomes jelly-like and turns into yoghurt finally. In short, what happens to bread and yogurt before they appear on our table is unbelievable. Especially when you think that the organisms that do it are unicellular.